Method of burning slag forming fuel in furnaces



Jan; 17, 1951 R. J. ZOSCHAK ET AL METHOD OF BURNING SLAG FORMING FUEL INFURNACES Filed March 18, 1959 DAMA/V ATTORNEY United States Patent()fiFice METHOD OF BURNING SLAG FORMING FUEL IN FURNACES Robert J.Zoschak, Rutherford, and Ernest L. Daman,

Westfield, N.J., assignors to Foster Wheeler Corporation, New York,N.Y., a corporation of New York Filed Mar. 18, 1959, Ser. No. 800,214

8 Claims. 01. 122-240 This invention relates to steam generators, moreparticularly to the method of burning slag forming fuels in largecapacity supercharged vapor generating power plants.

In large capacity supercharged vapor generating power plants, it iscommercially impractical to provide all the necessary heat transferelements in a single shell because of the construction problems arisingfrom the fact that combustion gas pressures in supercharged vaporgenerating plants are in the order of 60 to .80, p.s.i.g. Therefore,large capacity supercharged vapor generating plants comprise a pluralityof separate shells or sections, each of which contains heat transferelements and is independent- 1y fired to generate vapor and superheatvapor and/or reheat vapor. The combustion gas generated in each of theplurality of sections is conducted from each of the shells to a commonflue, and thence to a gas turbinecompressor assembly. These sections aregas or oil fired to obviate the problem of non-combustible matter whichis entrained in the combustion gas adhering to the surfaces of the heattransfer elements in the convection zone, which problem exists whereslag producing fuels are burned. It is well known in the boiler art thatslagging on the convection elements may be obviated where a slaggingfuel, such as pulverized coal, is burned by recirculating combustion gasfrom a point, with respect to the combustion gas flow, after theeconomizer of a boiler into the combustion zone or furnace cham ber toreduce the temperature of the combustion gas in the combustion zone sothat the combustion gas is below the fusion point of the ash before itreaches the convection elements. However, recirculation of combustiongas after the economizer section in a supercharged boiler is noteconomically feasible since the combustion gas must first pass through agas turbine where the combustion gas pressure is substantially reducedso that recirculation of combustion gas after the economizer wouldrequire an exceptionally large recirculation compressor which results ina substantial loss in overall efficiency of the power plant.

It is therefore one of the objects of this invention to provide a methodof burning a slag forming fuel in a multi-section supercharged vaporgenerator plant without a slagging condition occurring on the heattransfer elements in the convection zones of the sections. Anotherobject of this invention is to provide a method of burning a slagforming fuel in a supercharged multi-section vapor generating plantwhereby plant efficiency is superior to conventional non-superchargedvapor generators of comparable capacity.

Accordingly, the present invention contemplates, in a multi-sectionsupercharged vapor generating plant, a method of burning a slag formingfuel wherein a slag forming fuel is burned in a first combustion zone toproduce a combustion gas at a temperature above the fusion point of thenon-combustible material in the combustion gas so that thenon-combustible material is in a molten State, and then, flowing thefirst combustion gas from said first combustion zone into indirect heatexchange relationship with a relatively cool fluid to heat the latterand 0001 said first combustion gas. Thereafter, pumping a portion of thecooled first combustion gas into the first combustion zone and intoadmixture with the first combustion gas generated in the combustion zoneto produce a first resultant combustion gas having a temperature belowthe fushion point of the non-combustible material entrained in theresultant combustion gas so that the entrained molten non-combustiblesare solidified before the latter gas passes into indirect heat exchangerrelationship with said relatively cool fluid. In a second combustionzone, burning a slag forming fuel to produce a second combustion gashaving a temperature above the fusion point of the non-combustiblematerial in the second combustion gas so that the non-combustiblematerial is in a molten state. Thereafter, passing the other portion ofthe cooled first combustion gas into said second combustion zone andinto admixture with said second combustion gas to produce a secondresultant gas having a temperature below the fusion point of thenon-combustible material entrained in said second resultant combustiongas so that the entrained molten non-combustibles are solidified beforepassing from said second combustion zone. From the second combustionzone, the second resultant combustion gas is passed in indirect heatexchange relationship with a relatively cool fluid to heat the latterand cool the second resultant combustion gas. In a third combustionzone, burning a slag forming fuel to produce a third combustion gashaving a temperature above the fusion point of the non-combustiblematerial in the third combustion gas so that the non-combustiblematerial is in a molten state. Thereafter passing the cooled secondcombustion gas into said third combustion zone and into admixture withthe third combustion gas to provide a third resultant combustion gashaving a temperature below the fustion point of the non-combustiblesentrained in the third resultant gas so that the entrained moltennon-combustibles solidify before the latter combustion gas is passedfrom the third combustion zone. The third resultant combustion gas isthen passed into indirect heat exchange relationship with a relativelycool fluid to heat the'latter and cool the third resultant gas. Afterthe third resultant gas is cooled, it is passed to a place of use, suchas a turbine-compressor assembly, wherein the gas functions to turn theturbine which in turn operates the compressor to compress the combustionair which is fed to the first, second and third combustion zones.

The invention will be more fully understood from the followingdescription when considered in connection with the accompanying drawing.

matically shown. For illustration purposes the power plant is shown ascomprising three separate sections or boilers 10, 11 and 12 although itis contemplated that the power plant may comprise two boilers or morethan three sections or boilers without departing from the spirit andscope of this invention.

1 The boilers 10, 11 and 12 comprise cylindrical settings 13, 14 and 15,respectively, having in the lower portions thereof high temperaturecombustion zones or furnaces 16, 17 andv 18, respectively, and in theupper portions thereof convection 'zones or sections 19, 20 and 21,respectively. Each of the boilers 10, 11 and 12 is provided withsuitable fuel burners 22 which are connected to receive from a suitablesource thereof (not shown) a slag forming fuel, as for example,pulverized coal, and are Patented Jan. 17, 1961,

valved lines 23, 24 and 25, respectively. Valved lines 23, 24 and 25 areeach connected to a main combustion air supply line 26 which isconnected to receive compressed air .from a gas turbine-compressorassembly 27. In the convection zones or sections 19, 20 and 21 of theboilers, vapor generating tubes 28 may be arranged around the innersurfaces of the settings, which tubes may be connected at one end toannular inlet headers 29 and at the opposite ends to outlet headers 30.

A shown, boiler is provided, in convection section 19, with a secondarysuperheater tube bank 31 and, above the latter, a vapor generating tubebank 32. In convection section 20 of boiler 11, a primary superheatertube bank 33 is provided, while, in convection section 21 of boiler 12,a reheater tube bank 34 and a convection vapor generating tube bank 35are disposed. It is to be understood that while convection vaporgenerating tube banks 32 and 35, superheater tube banks 31 and 33, andreheater tube bank 34, are shown in the form of a helical coil, they maybe constructed and arranged within the convection sections in any othersuitable manner.

The boilers 10, 11 and 12 are interconnected, as hereinafter described,to a single vapor-liquid drum 36 although two or more vapor-liquid drumsmay be provided without departing from the spirit and scope of thisinvention.

Vapor generating tubes 28 of each of the boilers 10, 11 and 12 areconnected through inlet headers 29 and downcomers 37 to vapor-liquiddrum 36 to receive liquid from the latter. The liquid flows throughtubes 28 and in indirect heat exchange relationship with products ofcombustion flowing through convection sections 19, 20 and 21 of theboilers whereby saturated vapor is generated. The tubes 28 areconnected, through outlet headers 30 and risers 38, to vapor liquid drum36 to pass the saturated vapor to the latter. Convection vaporgenerating tube banks 32 and 35 of boilers 10 and 12 are connected tovapor-liquid drum 36 by way of downcomers 39. The liquid flows throughtube banks 32 and 35 and in indirect heat exchange relationship withcombustion gas flowing through the respective convection sections 19 and21 of boilers 10 and 12 to produce saturated vapor. Vapor generatingtube banks 32 and 35 are connected by risers 40 to vapor-liquid drum 36to pass the saturated vapor to the latter.

Primary superheater tube bank 33, in convection section 20 of boiler 11,is connected through line 41 to receive vapor from vapor-liquid drum 36,the vapor in flowing through the primary superheater passes in indirectheat exchange relationship with combustion gas flowing throughconvection section 21 and is thereby heated. Tube bank 33 is connectedby line 42 to secondary superheater tube bank 31, in convection section19 of boiler 10, to conduct the superheated vapor to superheater tubebank 31. In secondary superheater tube bank 31, the superheated vapor isheated to a final predetermined superheat temperature by passing inindirect heat exchange relationship with combustion gas flowing throughconvection section 19 of boiler 10. The superheated vapor is conductedfrom secondary superheater tube bank 31 to a high pressure stage of avapor turbine, not shown.

Reheater tube bank 34, in convection section 21 of boiler 12, isconnected through a line 43 to an intermediate pressure stage of a vaporturbine (not shown) to receive vapor therefrom. The vapor is heated inreheater tube bank 34 to a predetermined temperature by passing inindirect heat exchange relationship with combustion gas flowing throughconvection section 21. The reheated vapor is conducted from reheatertube bank 34 by way of line 44 to a low pressure stage of a vaporturbine, not shown.

As previously described, each of the boilers 10, 11 and 12 are providedwith fuel burners 22 which are adapted to burn a slag forming fuel inthe respective combustion. zones 16, 17 and 18, of boilersto therebypro-- duce, in each of the combustion zones, a combustion gas having atemperature above the fusion point of the noncombustible material in thefuel. To obviate slagging of the non-combustible material entrained inthe combustion gas on the heat transfer elements which are disposed inconvection sections 19, 20, 21 of the boilers, the boilers are seriallyinterconnected for the flow of cooled combustion gas into theirrespective combustion zones as hereinafter described.

A line or duct 45 is connected at one end to the top portion of boiler10 and in communication with convection section 19 to receive from thelatter combustion gas which has been cooled by flowing over secondarysuperheater tube bank 31, vapor generating tube bank 32, and adjacentvapor generating tubes 28, duct 45 being connected at its other end totwo branch ducts 46 and 47. Duct 46 communicates through a recirculationfan or blower 48 with combustion zone 16 of boiler 10, while duct 47communicates with combustion zone 17 of boiler 11. Recirculation blower48 draws a portion of the cooled combustion gas, through ducts 45 and46, and forces a sufiicient amount of the cooled combustion gas intocombustion zone 16 and into admixture with the combustion gas generatedwithin the combustion zone to produce a combustion gas mixture having atemperature below the fusion point of the molten non-combustiblematerial in the combustion gas so that the entrained non-combustiblematerial solidifies before the combustion gas mixture passes intoconvection section 19 of boiler 10. The other portion of the cooledcombustion gas flows through ducts 45 and 47 into combustion zone 17 ofboiler 11 and into admixture with the combustion gas generated incombustion zone 17 to produce a combustion gas mixture having atemperature below the fusion point of the molten non-combustible materalin the combustion gas so that the entrained non-combustible materialsolidifies before the combustion gas mixture passes into the convectionsection 20 of boiler 11.

A duct 49 is provided to pass the combustion gas mixture which has beencooled in passing in indirect heat exchange relationship with the vaporgenerating tubes 28 and primary superheater tube bank 33 in convectionsection 20 of boiler 11, from the top portion of convection section 20,into combustion zone 18 of boiler 12 and into admixture with thecombustion gas generated in combustion zone 18 to produce a combustiongas mixture having a temperature below the fusion point of the moltennon-combustible material in the combustion gas so that the entrainednon-combustible material solidifies before the combustion gas mixturepasses into convection zone 21. A duct 50 is provided to conduct thecombustion gas which has been cooled by passing in indirect heatexchange relationship with the fluid in vapor generating tubes 28, vaporgenerating tube bank 35 and reheater tube bank 34, from the top portionof convection section 21 to gas turbine-compressor assembly 27, thecombustion gas driving the turbine which in turn drives the compressor.

By the relative adjustment of the valves in valved lines 23, 24 and 25to control flow of combustion air into the respective combustion zonesof boilers 10, 11 and 12, and by differential firing of the boilers, alower combustion gas pressure is maintained in boiler 11 than in boiler10 and a still lower combustion gas pressure in boiler 12 than in boiler11. By maintaining a combustion gas pressure differential between boiler10 and boiler 11 and between boiler 11 and boiler 12, cooled combustiongas will flow from convection section 19 of boiler 10 through ducts '45and 47, into combustion zone 17 of boiler 11, without the need for acirculating fan or blower. Likewise, combustion gas will flow fromconvection section 20 of boiler 11 into combustion zone 18 of boiler 12through duct 49 without the need for a circulating fan or blower becauseof the differential combustion gas pressurewhich is. maintained betweenboilers 11 and 12. A

blower 48 is required to effect recirculation of a portion of thecombustion gas from convection section 19 into combustion zone 16through ducts 45 and 46 since there is a small pressure drop throughconvection section 19 of boiler and duct 45.

However, while it is desirable-to maintain a combust1on gas pressuredifferential between boilers 10, 11 and 12, it is essential for maximumefficiency of the supercharged vapor generating power plant to maintainas small differential combustion gas pressure as possible between theboilers, as for example, a combustion gas pressure differential betweenboiler 10 and boiler 12 within the range of 2 to 4 p.s.i.g.

In operation of the supercharged vapor generating power plant abovedescribed, a slag forming fuel, such as pulverized coal, is fired in therespective high temperature combustion zones 16, 17 and 18 of boilers10, 11 and 12 by fuel burners 22 to generate combustion gas having atemperature within the range of 2,600 F. to 3,000 E, at whichtemperature the non-combustible materials in the fuel are in a moltenstate. While only one burner is shown in each of the boilers, it iscontemplated that a plurality of burners may be employed which arearranged and directed to produce a cyclonic effect in combustion zones16, 17 and 18.

A substantial amount of the molten non-combustible material separatesfrom the gaseous products of combustion in combustion zones 16, 17 and18 and flows downwardly along the walls of the combustion zones and outof the bottom of the boilers through an opening therein, not shown, andoutlet means 16A, 17A and 18A of the respective boilers 10, 11 and 12.The combustion gas in each of the combustion zones 16, 17 and 18 passesupwardly into the respective convection sections 19, 20 and 21 and inindirect heat exchange relationship With the fluids flowing through theheat transfer elements disposed therein to heat the latter and cool thecombustion gas as hereinbefore described. Secondary superheater tubebank 31, vapor generating tube bank 32 and vapor generating tubes 28 inconvection section 19 of boiler 10 are constructed and arranged inrelation to combustion gas temperature and flow rates, to provide a rateof heat transfer suflicient to cause the combustion gas flowing throughconvection section 19 to be cooled to a temperature of approximately 900F., while primary superheater tube bank 33 and vapor generating tubes 28in convection section 20 of boiler 11 are constructed and arranged inrelation to combustion gas temperature and flow rates, to provide a rateof heat transfer sufiicient to cause the combustion gas passing throughconvection section 20 to be cooled to a temperature of 1,000 F. Inconvection section 21 of boiler 12, reheater tube bank 34, convectionvapor generating tube bank 35 and vapor generating tubes 28 areconstructed and arranged to provide a heat transfer rate in relation tothe combustion gas temperature and flow rates, which will cool thecombustion gas, flowing through convection section 21 to a temperatureof 1,4Q0 F. a I

To prevent slagging of the molten non-combustible material entrained inthe combustion gas, a portion of the cooled combustion gas flows fromconvection section 19 of boiler 10 by Way of duct 45 and is pumped byblower 48 into the combustion zone 16 of boiler 10 by way of line 46.while the other portion of the cooled combustion gas flows through duct'47 into the combustion zone17 of boiler 11. At the same time, thecooled combustion gas passes from convection section 20 of boiler 11through duct 49 into combustion zone 13 of boiler 12.

To insure optimum mixing of the cooled combustion gases with the hightemperature combustion gases in the respective combustion zones 16, 17and '18 of boilers 10, 11 and 12, so that substantially all of thenon-combustible material entrained in the combustion gases is-solidified prior to passage into the respective convection zones 19,,20 and 21, each of the ducts 46, 47 and 49 may be connected tomanifolds or gas plenum chambers (not shown) which are disposed adjacentthe combustion zones so that the cool combustion gases enter thecombustion zones at various spaced points and at such angles as toachieve complete mixing with the high temperature combustion gases.

The cooled combustion gases from convection zone 21 of boiler 12 passthrough duct 50 into the turbine of turbine-compressor assembly 27 and,after discharge from the turbine, pass to a suitable economizer and/orair heater (not shown) through line 51.

From the foregoing description, it can be readily seen that a novelmethod of gas recirculation has been provided in a multi-boilersupercharged vapor generating power plant whereby a slag forming fuelmay be burned without slagging of non-combustible material on the heattransfer elements in the convection section of the boilers. It is amethod of recirculating cool combustion gases into the combustion zonesof the boilers where only one recirculation fan or blower of relativelysmall size is required.

Although but one embodiment of the invention has been illustrated anddescribed in detail, it is to be ex pressly understood that theinvention is not limited thereto. Various changes may be made in thedesign and arrangement of parts without departing from the spirit andscope of the invention, as the same will now be understood by thoseskilled in the art.

What is claimed is:

1. In a supercharged vapor generating plant having a plurality ofcombustion zones, the method of firing a slag forming fuel comprisingthe steps of burning a slag forming fuel in a first combustion zone toproduce a first combustion gas, passing said combustion gas intoindirect heat exchange relationship with heat transfer elements wherebyheat is absorbed from said gas, recirculating a portion of said firstcombustion gas after it is cooled into the first combustion zone andinto admixture with the combustion gas generated in said firstcombustion zone by the burning of slag forming fuel, burning a slagforming fuel in a second combustion zone to produce a second combustiongas, passing the other portion of the cooled first combustion gas intosaid second combustion zone and into admixture with said secondcombustion gas, passing the admixed first and second combustion gasesinto indirect heat exchange relationship with heat transfer elementswhereby heat is absorbed from said gases, burning a slag forming fuel ina third combustion zone to produce a third combustion gas, flowing thecooled first and second combustion gas admixture into said thirdcombustion zone and into admixture with said third combustion gas,passing the first, second and third combustion gas admixture in indirectheat exchange relationship with heat transfer elements whereby heat isabsorbed from said combustion gas admixture and thereafter passing thecooled first, second and third combustion ga's admixture to a place ofuse.

2. In a supercharged vapor generating plant, the method of burning aslag forming fuel comprising the steps of burning a slag forming fuel ina first combustion zone to produce a first combustion gas, passing saidfirst combustion gas in indirect heat transfer relationship withrelatively cool fluid to heat the latter and cool said first combustiongas, recirculating a portion of said cooled first combustion gas intosaid first combustion zone and into admixture with the combustion gasgenerated in said first combustion zone by the burning of slag formingfuel, burning a slag forming fuel in a second combustion zone to producea second combustion gas, passing the other portion of said cooled firstcombustion gas into said second combustion zone and into admixture withsaid second combustion gas, then passing the admixture of said first andsecond combustion gases into indirect heat transfer relationship with arelatively cool fluid to heat the latter and cool said combustion gasadmixture, burn- 7 ing a slag forming fuel in a third combustion zone toproduce a third combustion gas, flowing the cooled first and secondcombustion gas admixture into said third combustion zone and intoadmixture with said third combustion gas, passing the first, second andthird combustion gas admixture in indirect heat transfer relationshipwith a relatively cool fluid, and thereafter passing the cooled first,second and third combustion gas admixture to a place of use.

3. In a supercharged vapor generating power plant, the method comprisingthe steps of burning a slag forming fuel in a first high temperaturecombustion zone to produce a first combustion gas at a temperature abovethe fusion point of the non-combustibles in said slag forming fuel,passing said first combustion gas in indirect heat exchange relationshipwith a relatively cool fluid to heat the latter and cool the firstcombustion gas, recirculating a portion of said cooled first combustiongas into'said first combustion zone and into admixture with sadi firstcombustion gas to provide a first resultant combustion gas having atemperature below the fusion point of the non-combustibles in said slagforming fuel so that substantially all of the molten non-combustiblesentrained in said resultant combustion gas mixture are solidified,burning a slag forming fuel in a second high temperature combustion zoneto produce a second combustion gas a temperature above the fusion pointof the non-combustibles in said slag forming fuel, passing the otherportion of said cooled first combustion gas into said second combustionzone and into admixture with said second combustion gas to provide asecond resultant combustion gas mixture having a temperature below thefusion point of the non-combustibles in said slag forming fuel tosolidify substantially all of the molten non-combustibles in the secondresultant combustion gas mixture before it passes from said secondcombustion zone, passing the second resultant combustion gas mixtureinto indirect heat exchange relationship with a relatively cool fluid toheat the latter and cool the second resultant combustion gas mixtureburning a slag forming fuel in a third high temperature combustion zoneto produce a third combustion gas at a temperature above the fusionpoint of the non-combustibles in said slag forming fuel, passing saidcooled second resultant combustion gas mixture into said thirdcombustion zone and into admixture with said third combustion gas toprovide a third resultant combustion gas mixture having a temperaturebelow the fusion point of the non-combustibles in the slag forming fuelto solidify substantially all of the molten non-combustibles in thethird resultant combustion gas mixture, passing the third resultantcombustion gas mixture into indirect heat exchange relationship with arelatively cool fluid to heat the latter and cool the third resultantcombustion gas mixture, and thereafter passing said cooled thirdresultant combustion gas mixture to a place of use.

4. In the supercharged vapor generating power plant, the method ofburning slag forming fuel comprising the steps of burning a slag formingfuel in a first high temperature combustion zone to produce a firstcombustion gas at a temperature above the fusion point of thenoncombustibles in said fuel, passing said first combustion gas from thefirst combustion Zone into indirect heat exchange with fluid to heat thelatter and cool said first combustion gas, recirculating a portion ofcooled first combsution gas into said first combustion zone and intoadmixture with said first combustion gas at a point before the latterpasses into indirect heat exchange relationship with said fluid toproduce a first resultant combustion gas having a temperature below thefusion point of the non-combustibles in said fuel so that the entrainednoncombustibles are solidified before passing from said first combustionzone, burning a slag forming fuel in a second high temperaturecombustion zone to produce a second combustion gas at a temperatureabove the fusion point of the non-combustibles in said fuel, passing theother portion of said cool first combustion gas into said secondcombustion zone at a point before the second combustion gas passes fromsaid second combustion zone and into admixture with the latter toproduce a second resultant combustion gas having a temperature below thefusion point of the non-combustibles in said fuel so that the entrainednon-combustibles are solidified before passing from said secondcombustion zone, flowing said second resultant combustion gas from saidsecond combustion zone into indirect heat exchange relationship with afluid to heat the latter and cool said second resultant combustion gas,burning a slag forming fuel in a third high temperature zone to producea third combustion gas at a temperature above the fusion point of thenon-combustibles in said fuel, passing said cooled second combustion gasinto said third combustion zone at a point before the third combustiongas passes from said third combustion zone and into admixture with thelatter to produce a third resultant combustion gas having a temperaturebelow the fusion point of the noncombustibles in the fuel so that theentrained non-combustibles are solidified before passing from said thirdcombustion zone, passing the third resultant combustion gas from saidthird combustion zone into indirect heat exchange relationship with afluid to heat the latter and cool said third resultant combustion gas,passing the cooled third resultant combustion gas to a place of use.

5. In a supercharged vapor power plant having a plurality of combustionzones, the method of burning pulverized fuel in each of said zonescomprising the steps of burning a pulverized fuel in a first hightemperature combustion zone to produce a first combustion gas at atemperature above the fusion point of the non-combustibles in said fuel,passing said first combustion gas in indirect heat exchange relationshipwith a fluid to heat the latter and cool said first combustion gas,recirculating a portion of said first combustion gas into said firsthigh temperature combustion zone and into admixture with further firstcombustion gas generated in said combustion zone by the burning ofadditional quantities of pulverized fuel, the recirculation of saidportion of said first combustion gas into admixture with said furtherfirst combustion gas to produce a first resultant combustion gas havinga temperature below the fusion point of slag in said fuel so that theentrained molten slag is solidified before passing in indirect heatexchange relationship with said relatively cool fluid, passing saidfirst resultant combustion gas in indirect heat exchange relationshipwith fluid to heat the latter and cool said first resultant combustiongas, passing a portion of said cooled first resultant combustion gasinto said first combustion zone, burning pulverized fuel in a secondhigh temperature combustion zone to produce a second combustion gas at atemperature above the fusion point of the slag in said fuel, passing'theother portion of said cooled first resultant combustion gas into saidsecond high temperature combustion zone and into admixture with saidsecond combustion gas to produce a second resultant combustion gashaving a temperature below the fusion point of slag in said fuel tosolidify the entrained molten slag before said second resultantcombustion gas passes from said second combustion zone, flowing saidsecond resultant combustion gas from the second combustion zone intoindirect heat exchange relationship with a fluid to heat the latter andcool said second resultant combustion gas, burning pulverized fuel in athird high temperature combustion zone to produce a third combustion gasat a temperature above the fusion point of slag in said fuel, passingsaid cooled second resultant combustion gas into said third combustionzone and into admixture with said third combustion gas to produce athird resultant combustion gas having a temperature below the fusionpoint of said fuel so that said third resultant combustion gas is belowthe fusion point of said slag to solidify the entrained molten slagbefore passing from said third combustion zone, flowing said thirdresultant combustion gas into indirect heat exchange relationship with afluid to heat the latter and cool said third resultant combustion gas,and passing said cooled third resultant combustion gas to a place ofuse.

6. In a supercharged vapor generating plant having a plurality ofboilers each of which has a combustion zone and a convection zonedisposed thereabove, the method of burning a slag forming fuel in eachof said boilers comprising the steps of burning a slag forming fuel in afirst combustion zone of a first boiler to produce a first combustiongas at a temperature above the fusion point of the non-combustibles insaid fuel so that the noncombustibles are in a molten state, passingsaid first combustion gas from the first combustion zone into indirectheat exchange relationship with a fluid in a first convection zone ofsaid boiler to heat the fluid and cool said first combustion gas,passing the cooled first combustion gas from the first convection zone,passing a portion of said cooled first combustion gas into said firstcombustion zone and into admixture with the first combustion gas toproduce a first resultant combustion gas having a temperature below thefusion point of the noncombustibles in said fuel so that when the firstresultant combustion gas enters the first convection zone the entrainedmolten non-combustibles are solidified, burning a slag forming fuel in asecond combustion zone in a second boiler to produce a second combustiongas having a temperature above the fusion point of non-combustibles insaid fuel so that the non-combustibles are in a molten state, flowingthe other portion of said cooled first combustion gas into said secondcombustion zone of said second boiler to produce a second resultantcombustion gas having a temperature below the fusion point of thenon-combustibles in said fuel so that the entrained non-combustibles aresolidified, passing said second resultant combustion gas in a secondconvection zone in said second boiler and into indirect heat exchangewith a fluid to heat the latter and cool said second resultantcombustion gas, and passing said cooled second resultant combustion gasto a place of use.

7. In a supercharged steam generating plant having a plurality ofboilers each of which have a combustion zone and a convection zonedisposed thereabove, the method of burning a slagging fuel in each ofsaid boilers comprising the steps of burning a slag forming fuel in afirst combustion zone of a first boiler having a relatively highpredetermined combustion gas pressure to produce a first combustion gashaving a temperature above the fusion point of the non-combustibles insaid combustion gas so that the non-combustibles are in a molten state,passing said first combustion gas from said combustion zone into a firstconvection zone in said boiler and into indirect heat exchangerelationship with a relatively cool fluid to heat the latter and coolsaid first combustion gas, passing said cooled first combustion gas fromthe first convection zone and dividing said cooled first combustion gasinto two streams, pumping one stream into said first combustion zone andinto admixture with the first combustion gas generated in said firstcombustion zone to produce a first resultant combustion gas having atemperature below the fusion point of the non-combustibles in said firstcombustion gas so that said entrained molten non-combustibles aresolidified before the first resultant combustion gas passes into saidfirst convection zone, burning a slag forming fuel in a second hightemperature combustion zone of a second boiler having a combustion gaspressure lower than the predetermined pressure in said first boiler toproduce a second combustion gas having a temperature above the fusionpoint of the non-combustibles in said combustion gas so that thenoncombustibles are in a molten state, flowing the other stream of saidcooled first combustion gas into said second combustion zone and intoadmixture with said second combustion gas to produce a second resultantcombustion gas mixture having a temperature below the fusion point ofthe non-cornbustibles in the fuel so that the entrained moltennon-combustibles are solidified before said second resultant combustiongas passes from said second combustion zone, passing said secondresultant combustion gas from said second combustion zone into a secondconvection zone of said second boiler and into indirect heat exchangerelationship with a relatively cool fluid to heat the latter and to coolsaid second resultant combustion gas, burning a slag forming fuel in athird high temperature combustion zone of a third boiler having acombustion gas pressure lower than said second boiler to produce a thirdcombustion gas having a temperature above the fusion point of thenon-combustibles in said fuel so that the non-combustibles are in amolten state, passing the cooled second resultant combustion gas fromthe second convection zone into said third combustion zone and intoadmixture with the third combustion gas generated therein to produce athird resultant combustion gas having a temperature below the fusiontemperature of the non-combustibles in the fuel so that the entrainedmolten non-combustibles are solidified before said third resultant gaspasses from said third combustion zone, passing said third resultantcombustion gas from the third combustion zone into a third convectionzone and into indirect heat exchange relationship with a relatively coolfluid to heat the latter and cool the third resultant combustion gas,and thereafter flowing the cooled third resultant combustion gas fromsaid third convection zone to a place of use.

8. The method of claim 7 wherein the molten noncombustibles notentrained in the combustion gases in each of the combustion zones arewithdrawn from the combustion zones in a molten state.

References Cited in the file of this patent UNITED STATES PATENTS2,614,541 Armacost Oct. 21, 1952 2,641,233 Hemenway et al. June 9, 19532,818,837 Frisch Jan. 7, 1958 2,865,344 Firl Dec. 23, 1958 UNITED STATESPATENT QFFICE CERTIFICATION OF CORRECTION Patent No; 2,968 288 January17 1961 Robert J. Zoschak, e1; ale

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 8, for "fushion" read fusion line 37, for "fustion" readfusion column 4 line 35, for "materal" read material column .7 line 20for "sadi" read said line 65 for combsuti-on",\ firet dccurrence readcombustion Signed and sealed this 6th day of June 1961.,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

